Resistor element, stress sensor, and method for manufacturing them

ABSTRACT

A stress sensor in which the direction and magnitude of a stress being applied to a post bonded to or integrated with an insulating board can be grasped from variation in the resistance of resistor elements being stimulated by application of the stress while suppressing variation in the shape of each resistor. The resistor element comprises a resistor formed, by screen print, between a pair of electrodes for the resistor element, i.e. circuit pattern electrodes, arranged on the surface of the insulating board. The electrode is connected, through a conductor, with a board terminal part arranged at one end of the insulating board. The electrode and the conductor or a print accuracy adjusting member have a constant height from the surface of the insulating board. Arrangement of the conductor, electrode and print accuracy adjusting member is entirely identical or similar for the resistor elements in the vicinity thereof.

TECHNICAL FIELD

The present invention relates to resistor elements and stress sensors asone application filed thereof, which can be used, for example, for apointing device for personal computers or a multifunctional andmultidirectional switch for various electronic devices.

BACKGROUND ART

A stress sensor has been disclosed in Japanese Unexamined PatentApplication Publication No. 2000-267803, which is capable of graspingthe direction and magnitude of a stress applied to a post bonded to orintegrated with a surface of an insulating board from variation inresistance of a plurality of resistor elements caused by stimulationprovided thereto resulting form the application of the stress to thepost. The formation of the resistor elements thus disclosed, whichelements form a strain gage, is performed by screen printing of allconstituent elements of the resistor elements on a surface of a ceramicboard.

The structure is formed as shown in FIG. 15, in which four resistorelements 22 are disposed on two lines, which are along a surface of aninsulating board 20 and perpendicularly intersect each other at thecenter of a surface the insulating board 20, and are provided atsubstantially the same distance from the intersecting point. Inaddition, in the structure described above, a post 30 having a squarebottom surface is bonded so that the center thereof coincides with thecenter of the surface of the insulating board 20 and that individualsides of the bottom surface of the post 30 oppose the respectiveresistor elements 22. In addition, board terminal parts 24 are providedat end portions along the entire periphery of the insulating board 20 atapproximately regular intervals. Since conductors (electrodes) to beconnected to the resistor elements 22 and the board terminal parts 24are formed on the surface of the insulating board 20 by a screenprinting method, they have a uniform (predetermined) height from thesurface of the insulating board 20.

In recent years, in addition to stress sensors having the structure inwhich all constituents for constituting resistor elements are formed byscreen printing on a surface of a ceramic board, development of stresssensors using an insulating board provided with conductors, which areobtained by partly removing a conductor layer on a surface as remains,has also been carried out. The reasons for this is that, compared tothick film techniques such as a screen printing technique, theconductors of the insulating board described above can be easilyprocessed to have fine patterns, and in addition, an advantage of lowmanufacturing cost can also be obtained.

However, when the insulating board for a stress sensor is an insulatingboard having conductors which are obtained by partly removing aconductive layer on a surface as remains, and when the stress sensoruses parts of the conductors 9 as electrodes and resistor elements as astrain gage, each element formed of a resistor provided between arespective pair of the electrodes on the surface of the insulatingboard, a problem different from that of the conventional techniquedescribed above may arise.

In the past, the electrodes (conductors) constituting the resistorelements were formed by a screen printing method, and in the abovetechnique, the conductors are formed of the remains obtained by partlyremoving the conductor layer on the surface; hence, the problem isgenerated by the difference described above.

The difference described above is schematically shown in FIG. 7. FIG.7(a) is a schematic cross-sectional view of a resistor element 8 usingconductors (circuit pattern electrodes 1) as electrodes, which areobtained by partly removing a conductor layer on a surface of aninsulating board 3. FIG. 7(b) is a schematic cross-sectional view of aresistor element 8 using conductors (resistor-element electrodes(hereinafter referred to as “thick electrodes)) obtained by screenprinting, which is one of thick film techniques.

The conductor height shown in FIG. 7(a) mostly depends on the thicknessof the conductor layer which is originally disposed on a surface of theinsulating board 3 and is formed of copper or the like. In general, thisthickness is approximately from 18 to 36 μm. In addition, when theinsulating board 3 is a so-called double-sided board, in which theconductors 9 on both surfaces of the insulating board 3 are connected toeach other through a conductive material formed inside a through-hole byplating, the conductive material may be further adhered to theconductors 9 by this plating, and as a result, the height thereof may befurther increased to approximately 40 to 70 μm in some cases. On theother hand, the thickness of a thick film electrode 13 shown in FIG.7(b) can be determined optionally to some extent, and is generally setto approximately 10 μm.

Next, the difference in cross-sectional shape between the circuitpattern electrode 1 and the thick film electrode 13 will be described.The circuit pattern electrode 1 has a cross-sectional shape similar to arectangular shape, and it is understood that the circuit patternelectrode 1 has surfaces approximately perpendicular to that of theinsulating board 3 (FIG. 7(a)). On the other hand, the thick filmelectrode 13 has a curved cross-sectional shape primarily formed ofcomponents inclined with respect to the surface of the insulating board3, and it is understood that the thick film electrode 13 is primarilycomposed of surfaces smooth with respect to the surface of theinsulating board 3 (FIG. 7(b)).

Due to the difference between the circuit pattern electrode 1 and thethick film electrode 13, compared to the resistor element 8 (FIG. 7(b))using the thick film electrodes 13 as the electrodes, the resistorelement 8 (FIG. 7(a)) using the circuit pattern electrodes 1 as theelectrodes has a large variation in resistance. The reason for this isthat it is difficult for the former to form a resistor 2 having auniform shape. When the variation in resistance is large, in a so-calledtrimming step in which adjustment is performed to obtain a desiredresistance, a resistor element 8 in which an excessively long trimminggroove must be formed and a resistor element 8 in which a trimminggroove is not substantially necessary are both present at the same time.Although the resistances are equal to each other, when the trimminglengths are extremely different from each other as described above, dueto the change in ambient environment, particularly an ambienttemperature, the resistance stability cannot be obtained. That is, evenwhen the nominal resistances are the same, in this case, resistorelements 8, in which the variation in various properties other than theresistance is large, are formed. In addition, in a stress sensor inwhich resistor elements 8 having trimming grooves are used as a straingage, minute cracks around the trimming grooves will grow by the use fora long period of time, and as a result, the original resistance maynot-be maintained in some cases.

As described above, compared to the case in which the thick filmelectrodes 13 are used, when the circuit pattern electrodes 1 describedabove are used, the shape of the resistor 2 having a large thicknessformed between the electrodes by a thick film technique such as a screenprinting technique becomes unstable, and it has been believed that thereare two reasons for this problem.

The first reason is that the height of the circuit pattern electrode 1is large as described above. For example, in the case in which a filmfor the resistor 2 is formed by a screen printing method, anapproximately predetermined amount of a resistor paste which passedthrough a mask (screen) is supplied between a pair of the circuitpattern electrodes 1. Depending on various factors such as an ambienttemperature, a paste temperature, and a holding time for fixing theshape of the resistor 2 obtained by firing or curing performed afterscreen printing, the shape of the fixed resistor 2 varies. For example,due to a high ambient temperature or the like, when the paste viscosityis low, the upper surface of the resistor 2 between a pair of thecircuit pattern electrodes 1 becomes approximately flat, and as aresult, a relatively stable shape is obtained. On the other hand, when apaste having a high viscosity is supplied between a pair of the circuitpattern electrodes 1, the paste is solidified by firing/curing whilemaintaining the original shape, which is formed when the paste issupplied, to some extent. It has been believed that this phenomenonbecomes apparent when the resistor paste contains a thermosetting resin.The reason for this is believed that decrease in paste viscosity is notlikely to occur even when the paste is heated. When the height of thecircuit pattern electrode 1 is large, an area around the circuit patternelectrode 1 becomes a paste easy-flow region when the viscosity of theresistor paste is high. The reason for this is that the paste in thevicinity of the peak of the circuit pattern electrode 1 moves from ahigher position to a lower position by its own weight.

In addition, in the case in which the film for the resistor 2 is formedby a screen printing method and in which the height of the circuitpattern electrode 1 is excessively large, when the resistor paste isallowed to pass through a mask by a squeegee, the squeegee is likely tocollide against the circuit pattern electrode 1. Hence, the squeegeesupplies the resistor paste through the mask in an irregular manner,resulting in the variation in amount of the resistor paste suppliedthrough the mask and, in addition, in the deviation of the position atwhich the resistor paste is supplied. Accordingly, the phenomenon inwhich the shape of the resistor 2 formed between the circuit patternelectrodes 1 is unlikely to be stable becomes more serious.

The second reason is that the circuit pattern electrode 1 has a surfaceapproximately perpendicular to the surface of the insulating board 3. Ithas been very difficult to control the resistor 2 present on theapproximately perpendicular surface to have a predetermined thickness.The reason for this is that, as described above, when the paste in thevicinity of the peak of the circuit pattern electrode 1 moves from ahigher position to a lower position by its own weight, it is difficultto estimate how the paste moves along the approximately perpendicularsurface. In addition to the presence of the approximately perpendicularsurface, the second reason described above makes the shape of theresistor 2 unstable in combination with the first reason. That is, whenthe height of the circuit pattern electrode 1 is small, the distance issmall along which the paste in the vicinity of the peak of the circuitpattern electrode 1 described above moves by its own weight from ahigher position to a lower position, and as a result, the variation inresistance caused by the difference of the thickness of the resistor 2on the approximately perpendicular surface from that in the directionperpendicular thereto is small enough to be ignored.

This second reason is not only applied to the film formation of theresistor 2 by a thick film technique such as screen printing but is alsoapplied to that of the film of the resistor 2 for the resistor element 8by a thin film technique such as sputtering. The reason for this isthat, for example, when sputtering is performed for the circuit patternelectrode 1 having a large height and an approximately perpendicularsurface, it is difficult to control the thickness of the resistor 2adhered to this perpendicular surface to be a predetermined value. Thatis, even in the film formation of the resistor 2 by a thin filmtechnique, it is difficult to control the shape of the resistor 2 to beuniform, and as a result, the variation in resistance is liable tooccur.

Accordingly, an object of the present invention is to decrease thevariation in resistance of a resistor element having a resistor filmformed between a pair of electrodes on a surface of the insulating board3, the electrodes being parts of conductors obtained as the remains bypartly removing a conductor layer on the surface of the insulatingboard. In addition, the present invention provides a stress sensor usingthe resistor elements described above.

DISCLOSURE OF INVENTION

Referring to FIG. 1, hereinafter, stress sensors having structures la toid of the present invention will be described. In order to achieve theobjects described above, the stress sensor having structure la of thepresent invention is a stress sensor in which the direction andmagnitude of a stress applied to a post 6 bonded to or integrated with asurface of an insulating board 3 can be grasped from variation inresistance of a plurality of resistor elements 8 caused by stimulationresulting from the application of the stress to the post 6. In thestress sensor described above, the resistor elements 8 are each composedof a resistor 2 formed by a screen printing method between a pair ofresistor-element electrodes (circuit pattern electrodes 1); theresistor-element electrodes are connected to board terminal parts 5,provided at one end of the insulating board 3, through conductors 9; theresistor-element electrodes and the conductors 9 have a predeterminedheight from the surface of the insulating board 3; and for all theplurality of resistor elements 8, the arrangements of the conductors 9and the resistor-element electrodes, in the vicinities of the respectiveresistor elements, are equal or similar to each other.

In addition, in order to achieve the objects described above, the stresssensor having structure 1 b of the present invention is a stress sensorin which the direction and magnitude of a stress applied to the post 6bonded to or integrated with a surface of the insulating board 3 can begrasped from variation in resistance of the plurality of resistorelements 8 caused by stimulation resulting form the application of thestress to the post 6. In the stress sensor described above, the resistorelements 8 are each composed of the resistor 2 formed by a screenprinting method between a pair of the resistor-element electrodes(circuit pattern electrodes 1); the resistor-element electrodes areconnected to the board terminal parts 5, provided at one end of theinsulating board 3, through the conductors 9; the resistor-elementelectrodes and the conductors 9 or print accuracy adjusting members 7have a predetermined height from the surface of the insulating board 3;and for all the plurality of resistor elements 8, the arrangements ofthe conductors 9 and the resistor-element electrodes or the printaccuracy adjusting members 7, in the vicinities of the respectiveresistor elements, are equal or similar to each other.

According to structures 1 a and 1 b of the present invention, that is,for all the plurality of resistor elements, since the arrangements ofthe conductors 9 and the resistor-element electrodes (circuit patternelectrodes 1) or the print accuracy-adjustment members 7, in thevicinities of the respective resistor elements, are equal or similar toeach other, the conductors 9 and the resistor-element electrodes or theprint accuracy adjusting members 7, which form one stress sensor,provided on the entire insulating board 3 can be disposed incawell-balanced manner. Hence, uniform squeegee movement in screenprinting of the resistor 2 and uniform squeegee shape in supplying aresistor paste between a pair of the circuit pattern electrodes 1 on thesurface of the insulating board 3 can be obtained for each resistor 2.Accordingly, the variation in shape of the resistors 2 in one stresssensor can be suppressed, and as a result, the objects of the presentinvention can be achieved. A material for a general squeegee is arubber-based material, and the shape thereof is easily and elasticallychanged. Hence, the paste is allowed to pass through opening portions ofa screen.

FIG. 2(a) is a side view of a screen printing step when viewed from theside in the direction perpendicular to the squeegee movement. The stateof the screen printing step, at the same timing as that shown in FIG.2(a), is observed from between the screen and the insulating board 3 atan angle of 90° rotated along the surface thereof and is shown in FIG.2(b). In FIG. 2(b), when a pair of the circuit pattern electrodes 1,which are the resistor-element electrodes, at the right side arecompared with a pair of the circuit pattern electrodes 1 at the leftside, the conductors 9 and the print accuracy adjusting members 7 arenot present in the vicinity of the former, and on the other side, theyare present in the vicinity of the latter. Accordingly, when theresistors 2 are screen-printed between the former circuit patternelectrodes 1 and between the latter circuit pattern electrodes 1, thesqueegee movement is naturally changed, and in addition, when theresistor paste is supplied onto the surface of the insulating board 3,the squeegee shape is naturally changed between the former and thelatter. Hence, when structures 1 a and 1 b of the present invention areused, the arrangements of the conductors 9 and the print accuracyadjusting members 7 in the vicinities of the respective circuit patternelectrodes 1 can be made to be equal or similar to each other, and as aresult, uniform squeegee movement in screen printing of the resistor 2and uniform squeegee shape in supplying the resist paste onto thesurface of the insulating board 3 can be obtained.

The stimulation described above is elongation or contraction of theresistor elements 8, disposed on the insulating board 3, caused bywarping thereof, or compression or release of the compression of theresistor element 8 caused by the bottom surface of the post 6 withoutthrough the insulating board 3.

In general, the stress sensor comprises a control part in whichelectrical properties, such as the resistance described above, are forexample detected and computed, thereby functioning as a stress sensor.However, in this specification, for convenience, a portion excluding thecontrol part described above is referred to as a “stress sensor”.

In addition, “the post 6 is bonded to a surface of the insulating board3” indicates the state in which the post 6 and the insulating board 3are different members and are fixed together with an adhesive or thelike. In addition, “the post 6 is integrated with a surface of theinsulating board 3” indicates the state in which the post 6 and theinsulating board 3 are, for example, integrally formed. In thisspecification, the “outline of the bottom surface of the post 6” in thelatter case indicates a portion corresponding to that represented by the“outline of the bottom surface of the post 6” in the former case.

The resistor-element electrode described above is a material havingelectron conductivity and being in contact with the resistor 2 and ismade of part of the conductor 9 in many cases. For example, theresistor-element electrode is the circuit pattern electrode 1.

In the case in which the conductor 9 is formed by a thick film techniqueusing a screen printing method or the like, the predetermined heightdescribed above is several micrometers to ten and several micrometers.In the case in which the conductor 9 is formed by a thin film techniqueusing sputtering or the like, the predetermined height is approximatelyseveral tens nanometers. In addition, in the case in which a generalforming technique such as a subtract method or an additive method isused for forming the conductor 9 on a printed circuit board, thepredetermined height is several to several tens micrometers. Inaddition, since the height has a “predetermined” level, the case inwhich the conductor is buried in the surface of the insulating board 3is omitted. In addition, this “predetermined” height generally means a“uniform” height. That is, it means that, in one stress sensor, a largevariation in height of the conductors or the like is not present.

The “uniform” in this specification means substantial uniformity anddoes not means strict uniformity. For example, the variation in amountdeposited by plating is ignored. The advantage obtained from the“uniform” is that the squeegee movement becomes smoother in screenprinting.

In addition, concerning the term “one end”, in order to avoid themisunderstanding, that is, only one side forming the insulating board 3is regarded as the one end, generated from the narrow interpretation ofthe term, major portions of structures in which the board terminal parts5 are provided at one end of the insulating board 3 are shown in FIGS.6(a) to (g) by way of example. That is, the “one end” indicates arelatively small region along the entire periphery of the insulatingboard 3.

In addition, in the above “for all the plurality of resistor elements 8,and in the vicinities of”, the vicinity is a region which has a largeinfluence on the shape of the resistor 2 obtained by resistor 2formation using a screen printing method. In forming the resistor 2 byscreen printing, a region, in which a small variation in shape of theresistor 2 occurs and the influence thereof on the stress sensorproperties can be ignored, is not included in the vicinity.

In addition, the above “similar” is determined in principle inaccordance with the standard in which the influence on the stress sensorproperties can be ignored or not. However, shapes to be compared to eachother must be reasonably similar to each other. For example, thearrangements of the circuit pattern electrodes 1 or the print accuracyadjusting members 7 and the resistors 2 in the vicinities of the fourresistor elements 8, shown in FIG. 1, are similar to each other inappearance on the whole.

In addition, the print accuracy adjusting members 7 described above aremembers other than the conductors 9 and the resistor-element electrodes(circuit pattern electrodes 1) and are provided on the surface of theinsulating board 3 whenever necessary, together with the conductor 9 andthe resistor-element electrode, so as to obtain uniform squeegeemovement in forming the resistors 2 by screen printing and uniformsqueegee shape in supplying the resistor paste onto the surface of theinsulating board 3 for each resistor 2. The material therefore may be aconductive material or an insulating material.

The print accuracy adjusting members 7 are preferably formed togetherwith the conductors 9 and the resistor-element electrodes (circuitpattern electrodes 1) since approximately uniform height can beobtained, and the manufacturing can be easier performed. For example,when these three members are formed by screen printing, these threemembers are patterned (formation of opening portions) in one screenplate. In addition, when patterned by a so-called subtract method, thesethree members are arranged to be obtained by one etching operation as isthe case described above.

In addition, in order to achieve the objects described above, the stresssensor having structure 1 c of the present invention is a stress sensorin which the direction and magnitude of a stress applied to the post 6bonded to or integrated with a surface of the insulating board 3 can begrasped from variation in resistance of a plurality of the resistorelements caused by stimulation resulting from the application of thestress to the post 6. In the stress sensor described above, the resistorelements are composed of the resistors 2 formed by a screen printingmethod between pairs of the resistor-element electrodes (circuit patternelectrodes 1) disposed on a surface of the insulating board 3; theresistor-element electrodes are connected to the board terminal parts 5provided at one end of the insulating board 3 through the conductors 9;and the resistor-element electrodes and the conductors 9 or the printaccuracy adjusting members 7 have a predetermined height from thesurface of the insulating board 3; and for all the plurality of resistorelements, the arrangements of the conductors 9 and the resistor-elementelectrodes (circuit pattern electrodes 1) or the print accuracyadjusting members 7, in the vicinities of the respective resistorelements, are each-formed so as to surround at least three sides of theperiphery of each of the respective resistors 2.

The feature of structure 1 c of the present invention as compared tostructures 1 a and 1 b of the present invention described above is asfollows. In the latter structure, for all the plurality of resistorelements, the arrangements of the conductors 9, the resistance-elementelectrodes (circuit pattern electrodes 1) or the print accuracyadjusting members 7, in the vicinities of the respective resistanceelements, are equal or similar to each other, and on the other hand, inthe former structure, for all the plurality of resistor elements 8, thearrangements of the conductors 9, the resistance-element electrodes(circuit pattern electrodes 1) or the print accuracy adjusting members7, in the vicinities of the respective resistance elements, are eachformed so as to surround at least three sides of the periphery of eachof the resistors 2. The meanings of the terms, operations of theindividual constituent elements, and the like of the other points arecommon to all the structures. In addition, it is naturally understoodthat the combination of structure 1 c and structure 1 a or 1 b is notdenied. For example, the four resistor elements 8 shown in FIG. 1 havestructure 1 a, structure 1 b, and structure 1 c in combination.

The above “periphery of the resistor 2” is a region in the vicinity ofthe end portion of the resistor, which has a large influence on theresistor 2 shape formed by resistor 2 formation using a screen printingmethod, and including the outside of the vicinity described above. Theregion described above is, for example, approximately a region in thevicinity at which the resister-element electrodes (circuit patternelectrodes 1) are in contact with the resistor 2 shown in FIG. 1 or aregion including the outside of the region described above, that is, aregion in the vicinity at which the conductor 9 and the print accuracyadjusting member 7 are close to the resistor 2. In forming the resistor2 by screen printing, a region, which causes a small variation in shapeof the resistor 2 so that the influence thereof on the stress sensorproperties can be ignored, is not included in the region describedabove.

The uniform squeegee movement in forming the resistors 2 by screenprinting and the uniform squeegee shape in supplying a resistor pasteonto the surface of the insulating board 3 can be achieved for eachresistor 2 by using structure 1 c. The reason for this is that since thearrangements of the conductors 9 and the resistor-element electrodes(circuit pattern electrodes 1) or the print accuracy adjusting members 7each surround at least three sides of the periphery of each of therespective resistors 2, at least in the vicinities at which theresistors 2 are formed by printing, there are a great number of contactpoints between the squeegee and the conductors 9 and theresistor-element electrodes or the print accuracy adjusting members 7with a screen provided therebetween, the contact points beingcontinuously provided in many cases. As a result, the contact pointsdescribed above contribute to the improvement in uniformity of thesqueegee movement and the squeegee shape in supplying the resistor pasteonto the surface of the insulating board 3 for each resistor 2.

In addition, in order to achieve the objects described-above, the stresssensor having structure 1 d of the present invention is a stress sensorin which the direction and magnitude of a stress applied to the post 6bonded to or integrated with a surface of the insulating board 3 can begrasped from variation in resistance of a plurality of theresistor-elements 8 caused by stimulation, resulting from theapplication of the stress to the post 6. In the stress sensor describedabove, the resistor elements 8 are composed of the resistors 2 formed bya screen printing method between pairs of the resistor-elementelectrodes (circuit pattern electrodes 1) disposed on a surface of theinsulating board 3, the circuit pattern electrodes 1 are connected tothe board terminal parts 5 provided at one end of the insulating board 3through the conductors 9, and the circuit pattern electrodes 1 and theconductors 9 or the print accuracy adjusting members 7 have apredetermined height from the surface of the insulating board 3. Inaddition, the circuit pattern electrodes 1 and the conductors 9 or theprint accuracy adjusting members 7 are disposed so as to intermittentlyor continuously surround all the plurality of resistor elements.

The feature of structure 1 d of the present invention as compared tostructure 1 c of the present invention described above is as follows. Inthe latter structure, the conductors 9 and the resistor-elementelectrodes (circuit pattern electrodes 1) or the print accuracyadjusting members 7 are provided so as to surround the respectiveresistor elements, and on the other hand, in the former structure, theconductors 9 and the resistor-element electrodes or the print accuracyadjusting members 7 are provided so as to collectively surround theplurality of resistor elements. The meanings of the terms, operations ofthe individual constituent elements, and the like of the other pointsare common to all the structures. In addition, it is naturallyunderstood that the combination of structure 1 d and structures 1 aand/or structure 1 b and/or structure 1 c is not denied. The combinationdescribed above is more preferable since the advantages thereof may befavorably enhanced.

In these structures 1 a to 1 d described above, as the constituentelements of the stress sensors, the resistor-element electrodes (circuitpattern electrodes 1), the conductors 9, or the print accuracy adjustingmembers 7 are preferably formed by adhering a metal foil to the surfaceof the insulating board 3 followed by etching treatment performed forunnecessary parts of this metal foil. Compared to the case in which theresistor-element electrodes, the conductors 9, or the print accuracyadjusting members 7 are formed on the surface of the insulating board 3by a general thick film or thin film technique such as screen printingor sputtering, the height of the resistor-element electrodes, theconductors 9, or the print accuracy adjusting members 7 from the surfaceof the insulating board 3 is large as described above. The reason forthis is that the thicknesses thereof depend on the thickness of themetal foil described above, or that in an electroless plating step inwhich a conductive film is formed on inner walls of thorough-holes, theconductive film is also deposited on the metal foil. The thickness ofthe current metal foil is approximately 9 to 36 μm, and a foil having athickness of approximately 18 μm is generally used. When the electrolessplating step described above is performed, the height of the circuitpattern electrodes 1, the conductors 9, or the print accuracy adjustingmembers 7 from the surface of the insulating board 3 is generally 30 to50 μm. When the circuit pattern electrodes 1, the conductors 9, or theprint accuracy adjusting members 7, having a large height from thesurface of the insulating board 3, are used, it is particularlydifficult to obtain uniform squeegee movement in forming the resistors 2by screen printing and uniform squeegee shape in supplying the resistorpaste onto the surface of the insulating board 3 for each resistor 2,and hence, the application of the present invention can significantlycontribute to the improvement in stress sensor properties.

This significant contribution can be obtained when the height of theresistor-element electrodes (circuit pattern electrodes 1), theconductors 9, or the print accuracy adjusting members 7 is 10 μm ormore, more significant contribution can be obtained when the height is20 μm or more, and even more significant contribution can be obtainedwhen the height is 30 μm or more.

In addition, in order to achieve the objects described above, a methodfor manufacturing a stress sensor, according to the present invention,is a method for manufacturing a process sensor in which the directionand magnitude of a stress applied to the post 6 bonded to or integratedwith a surface of the insulating board 3 can be grasped from variationin resistance of a plurality of the resistor elements 8 caused bystimulation resulting from the application of the stress to the post 6.The method described above comprises a first step of forming the circuitpattern electrodes 1, the board terminal parts 5, and the conductors 9so that the resistor-element electrodes (circuit pattern electrodes 1)are connected to the board terminal parts 5 provided at one end of theinsulating board 3 through the conductors 9; a second step of providingan insulating film on a surface of the insulating board 3 so as not tocover at least the circuit pattern electrodes 1; and a third step offorming the resistors 2 by a screen printing method between pairs of thecircuit pattern electrodes 1 provided on the surface of the insulatingboard 3, wherein the first step, the second step, and the third step areperformed in that order.

The first step described above can be realized by a screen printingmethod in which a conductive paste is applied onto the surface of theinsulating board 3 formed of alumina or the like; a so-called subtractmethod in which a copper foil is adhered to a molded plate of a glassfiber filled epoxy resin, followed by etching to remove areas other thanthose necessary as the conductors 9; or a so-called additive method, aplating method, or the like in which the conductors 9 are deposited onnecessary areas.

In order to obtain the uniform squeegee movement in forming theresistors 2 by the screen printing in the subsequent third step and theuniform squeegee shape in supplying the resistor paste onto the surfaceof the insulating board 3 for each resistor 2, the second step describedabove is a step of adjusting the height of the resistor-elementelectrodes, the board terminal parts 5, and the conductors 9 from thesurface of the insulating board 3. That is, as described-above, when theheight of the resistor-element electrodes, the conductors 9, or theprint accuracy adjusting members 7 from the surface of the insulatingboard 3 is larger, in other words, when the difference in height of asurface of a workpiece, which is to be printed and to be brought intocontact with a squeegee for screen printing with a screen providedtherebetween, is larger, it becomes more difficult to obtain the uniformsqueegee movement. Accordingly, in order to decrease or eliminate thedifference in height described above, the level of the surface of theinsulating board 3 is increased so as to be closer to the height of theresistor-element electrode or the conductor 9 or to exceed the heightthereof by forming the insulating film over the conductors 9.

In the case in which a stress applied to the post 6 warps the insulatingboard 3, the resistor elements 8 are then warped thereby, and the stresssensor detects the variation in resistance of the resistor elements 8thus warped, the insulating film described above is preferably formed ofa material softer than the insulating board 3. The reason for this isthat when the insulating film is a material having high rigidity ascompared to that of the insulating board 3, the warping of theinsulating board 3 may be inhibited in some cases. For example, when thematerial for the insulating board 3 is a molded body of a glass fiberfilled epoxy resin, a cured silicone resin paste or the like may bepreferably used. In this case, for example, the paste is applied byscreen printing or the like so as to cover the surface of the insulatingboard 3 and the conductors 9 provided thereon. Accordingly, the paste onthe conductors 9, provided at the higher position, flows to the surfaceof the insulating board 3 located at the lower position and is thencured by heating to form an insulating film, and hence the difference inheight described above can be decreased or eliminated. In this step,attention must be paid so that the paste is not applied onto thesurfaces of the resistor-element electrodes (circuit pattern electrodes1). The reason for this is that the presence of a material which mayinterfere with the electrical connection with the resistors 2 formed inthe subsequent step is avoided. In this specification, the surface ofthe resistor-element electrodes includes the top surface and/or the sidesurfaces thereof. Hence, when the top surface of the resistor-elementelectrode is exposed, of course, the insulating film may be disposed insome cases between the electrodes at which the resistor 2 is to beprovided.

Means for not applying the paste on the surfaces of the circuit patternelectrodes 1 may comprise, for example, performing masking treatment inwhich the contact between the paste and the circuit pattern electrodes 1is inhibited, and removing the mask after the paste is cured.Alternatively, for example, after the paste is applied onto the surfacesof the circuit pattern electrodes 1 and curing thereof, the paste isremoved by polishing the surfaces of the circuit pattern electrodes 1.

A first structure of the resistor element 8 of the present invention,which achieves the objects described above, comprises: electrodes(circuit pattern electrodes 1) composed of parts of the conductors 9 ona surface of the insulating board 3 obtained by partly removing aconductive layer on the surface as remains or by an additive method; andthe resistor 2 formed by film formation between a pair of the circuitpattern electrodes 1 on the surface of the insulating board 3. In thestructure described above, the ratio L/h of the distance (L) between thepair of the electrodes to the electrode height (h) is 30 or more.

In FIG. 9, positions at which the distance (L) between the electrodesand the electrode height (h) are measured are shown. As means forobtaining a ratio L/h of 30 or more, for example, there may be mentionedmeans for decreasing the electrode height (h) and means for increasingthe distance (L) between the electrodes. In addition, of course, themeans described above may be used in combination.

By the means for decreasing the electrode height (h), the variation inresistance of the resistor elements 8 caused by the first reason and thesecond reason described above can be decreased. In addition, when aratio L/h of 30 or more is obtained by this means, even in the resistorelements 8, each comprising the electrodes (circuit pattern electrodes1) composed of parts of the conductors 9 obtained by partly removing theconductive layer on the surface as the remains or by an additive method;and the resistor 2 formed by film formation between the pair of thecircuit pattern electrodes 1 on the surface of the insulating board 3,the variation in resistance can be decreased.

When the electrode height (h) is decreased, in the structure in whichthe top surface of the circuit pattern electrode 1 is at the same levelof the surface of the insulating board 3 or in the structure in whichthe top surface of the circuit pattern electrode 1 is located at a lowerlevel than the surface of the insulating board 3, said h becomes 0 orless, and as a result, the ratio L/h cannot be 30 or more. However, evenin the case described above, since the same advantage as that of thefirst structure described above can be obtained, in the presentinvention, the case in which said h is 0 or less is also included in thestructure of the present invention.

In addition, in the case in which a ratio L/h of 30 or more is obtainedby the means for increasing the distance (L) between the electrodes, dueto the first and the second reasons described above, even when thevariation in shape of the resistor 2 in the vicinities of the circuitpattern electrodes 1 occurs, the variation can be decreased so as to beignored. That is, in each of the resistor 2 provided between the pair ofthe circuit pattern electrodes 1, when the ratio of part of the resistor2 provided at a relatively distant position from the surfaces of thecircuit pattern electrodes 1 and having a relatively reproducible shapeis increased, the variation in resistance of the resistor elements 8 canbe decreased. In other words, among the factors determining theresistance, including an unstable factor (part of the resistor 2provided in the vicinities of the circuit pattern electrodes 1) and astable factor (part of the resistor 2 provided at a distant from thesurfaces of the circuit pattern electrodes 1 and having a relativelyreproducible shape), when the ratio of the stable factors is increased,the variation in resistance of the resistor elements 8 can besuppressed.

In the resistor element 8 having the first structure of the presentinvention, the technical meaning in which the ratio L/h is set to 30 ormore is based on the experimental result. When the ratio L/h was set toapproximately 24, the variation in resistance of the resistor elements 8was in a range of ±17% (n=30). Hence, in the case in which the ratio L/hwas set to approximately 30, the variation in resistance of the resistorelements 8 was in a range of ±9% (n=30). In addition, when the ratio L/hwas set to approximately 40, 45, 50, 55, and 60, the variation inresistance is slightly decreased in that order; however, the variationdescribed above is not so significantly different from that obtainedwhen the ratio L/h was set to approximately 30. This is the process andthe reason for determining that “the ratio L/h is 30 or more”.

A method for manufacturing the resistor element having the firststructure of the present invention, for achieving the objects of thepresent invention, comprises a fourth step of obtaining the conductors 9on a surface of the insulating board 3, a fifth step of positivelyadjusting the height of parts or the entirety of the conductors 9, and asixth step of, by using parts of the conductors 9 as electrodes, formingthe resistor 2 by film formation between a pair of the electrodesprovided on the surface of the insulating board 3, in which the fourthto the sixth steps are performed in the numerical order. In the fifthstep of the method described above, the ratio L/h of the distance (L)between said pair of the electrodes to the height (h) of the conductors9 is set to 30 or more, or said h is set to 0 or less.

As described above, the fourth step is a step, for example, of obtainingthe conductor 9 layer on the surface of the insulating board 3 byremoving the conductor 9 layer on the surface thereof or by an additivemethod.

The above fifth step is performed, for example, by a press step ofpressing the surface of the insulating board 3. This step is a step ofobtaining a ratio L/h of 30 or more by forcedly pressing the circuitpattern electrodes 1, which is formed to have a large height, into theinsulating board 3 or deforming the circuit pattern-electrodes 1 itselfso that the electrode height (h) is finally adjusted to be smaller. Asthis press step, for example, there may be mentioned a press step ofpressing the entire surface of the insulating board 3 by roller press orpress with a pressure using a flat-plate having no concave portions as adie, or a press step of pressing only parts of the insulating board 3corresponding to the circuit pattern electrodes 1.

In addition, the above fifth step may be a step, for example, ofpolishing the surface of the insulating board 3 or performing acidtreatment thereof. This step is a step of finally decreasing the height(h) of the circuit pattern electrodes 1 through adjustment by mechanicalpolishing, for example, using an abrasive paper or by immersing theinsulating board 3 in an acidic solution for dissolution of a metal sothat the ratio L/h is set to 30 or more. In this step, when theinsulating board 3 is used having the structure in which conductors 9patterns on two surfaces of the insulating board 3 are connected,to eachother through a conductive material provided in through-holes, thethrough-hole portions are preferably masked so as not to be brought intocontact with the acidic solution for preventing the conductive materialin the through-holes from being excessively dissolved.

When the first structure of the resistor element 8 of the presentinvention comprises a potion at which circuit patterns on two surfacesof the insulating board 3 are connected to each other with a conductivematerial provided in a through-hole, and the resistor 2 formed by filmformation between a pair of electrodes on the surface of the insulatingboard 3, the electrodes formed of parts of the conductors 9 on thesurface thereof, the electrode height (h) may be particularly increasedin some cases, and hence the present invention is preferably used. Thereason the electrode height (h) may be increased is that in amanufacturing method of a so-called double-sided circuit board, in orderto form conductive layers on inner walls of through-holes of theinsulating board 3 so that wires on two surfaces thereof are connectedto each other, an electroless plating step is performed. As a result,the electroless plating layer thus formed is also deposited on portionswhich are to be formed into the circuit pattern electrodes 1.

The above fifth step including the plating step as described above maybe a plating step of plating inside the through-holes formed in theinsulating board 3 after the pair of the electrodes on the surfacethereof is covered. Next, the electrode height (h) is adjusted to besmall, and the ratio L/h is set to 30 or more.

In the present invention, of course, at least two of the fifth stepsdescribed by way of example may be used in combination.

In addition, in a second structure of the stress sensor of the presentinvention, the post 6 is bonded to or integrated with one of surfaces ofthe insulating board 3 forming all the resistor elements 8 having thefirst structure of the present invention, and the direction andmagnitude of a stress applied to the post 6 is grasped from thevariation in resistance of the resistor elements 8 resulting from theapplication of the stress.

In the stress sensor described above, for example, as shown in FIGS. 1and 8, the resistor elements 8 are provided on two lines, which arealong a surface of the insulating board 3 forming the resistor elements8 and perpendicularly intersect each other at the center of a sensoreffective region of a surface of the insulating board 3, and areprovided at substantially the same distance from the center, and thepost 6 is bonded to or integrated with a surface of the insulating board3 so that the center thereof substantially coincides with the center ofthe bottom surface of the post 6. Accordingly, the direction andmagnitude of a stress applied to the post 6 is grasped from thevariation in resistance of the resistor elements 8 caused by elongation,contraction, or compression thereof resulting from the application ofthe stress.

Referring to FIG. 8, an example of the structure of the stress sensoraccording to the present invention will be further described. Theinsulating board 3 is formed, for example, of a glass fiber filled epoxyresin. On the bottom surface of the insulating board 3, four pairs ofcircuit pattern electrodes 1 are provided, and the resistors 2 areprovided between the respective pairs of circuit pattern electrodes 1,thereby forming the resistor elements 8. The resistor elements 8 areprovided on two lines, which are along the surface of the insulatingboard 3 and intersect perpendicularly to each other, and are provided atsubstantially the same distance from the intersection described above.To the top surface of the insulating board 3, the post 6 is fixed withan adhesive or the like, in which the bottom surface of the post has anapproximately square outline. In this step, the center of the bottomsurface of the post 6 is provided so as to substantially coincide withthe center of the insulating board 3.

In addition, L-shaped holes 10 are formed in the insulating board 3 sothat the corners of the L-shapes face the center of the insulating board3. These holes 10 serve so as to allow the insulating board 3 to beeasily warped by a stress applied to the post 6 and to efficientlypropagate the stress to the individual resistor elements 8. That is, inthe case in which a stress is applied to the post 6 when the holes 10are not provided, in addition to insufficient warpage of the insulatingboard 3, the stress applied in an optional direction may also bepropagated to the resistor element 8 provided in a different directiontherefrom in some cases, and hence the holes 10 are preferably formed.

In addition, trimmable chip resistors 11 which are to be connected tothe respective resistor elements 8 in series are provided on the topsurface of the insulating board 3. The resistor elements 8 on the bottomsurface of the insulating board 3 and the trimmable chip resistors 11 onthe top surface of the insulating board 3 are electrically connectedthrough through-holes (via holes), not shown in the figure, formed inthe insulating board 3. When it is difficult to adjust the resistance ofeach resistor element 8 in a predetermined range, the trimmable chipresistor 11 is used so that the sum of the resistances of the resistorelement 8 and the trimmable chip resistor 11 is adjusted in apredetermined range by trimming the trimmable chip resistor 11 using alaser trimmer or the like. The electrical connection state of thetrimmable chip resistors 11 and the resistor elements 8 is shown in FIG.4 by way of example. Electrical signals from the stress sensor areoutput through the board terminal parts 5.

Support holes 12 are used for fixing the stress sensor to a housing ofan electronic device or the like. In the fixed state obtained thereby,the peripheral portions of the insulating board 3 outside the holes 10become non-deformable portions which are not substantially deformed evenwhen a stress is applied to the post 6, and the insides of the holes 10become deformable portions which are deformed when a stress is appliedto the post 6 so as to elongate and contract the resistor elements 8.The trimmable chip resistors 11 are preferably provided in thenon-deformable portions so that the resistances thereof are not variedby the influence of the deformation of the insulating board 3.

The meanings of the terms used for the stress sensor having the secondstructure are equivalent to those used for the stress sensors havingstructures 1 a to 1 d. In addition, of course,.the combination of thesecond structure and structures 1 a to 1 d is not denied. Thecombination described above is more preferable since the advantagesthereof may be favorably enhanced.

In the structure shown in FIG. 8, the holes 10, the support holes 12,and the trimmable chip resistors 11 are particularly optionalconstituent elements (not essential elements) of the stress sensorhaving the second structure. Even when those described above areincluded in the constituent elements, the shape of the hole 10 is notlimited to an L shape, and the positions at which the support holes 12are provided are not limited to the four corners of the insulating board3 having a square outline. The shape of the hole 10 may be optionallychanged, for example, into a circular, a square, or a rounded squareshape, in accordance with design limitation, required functions,applications, and the like. In addition, the support holes 12 may beprovided at an approximately center of each side of the squareinsulating board 3 shown in FIG. 8.

In the structure shown in FIG. 1 or 8, the bottom surface of the post 6and a part or the entirety of the resistor elements 8 may overlap eachother without the insulating board 3 provided therebetween. In thiscase, the post 6 and the resistor elements 8 are provided on the samesurface of the insulating board 3. This structure has an advantage inthat the sensibility of the resistor elements 8 can be enhanced. Thereason for this is that most of the stress applied to the post 6directly stimulates the resistor elements 8 from the bottom surface ofthe post 6 not through the insulating board 3. As a result of thisstimulation, the resistor 2 portions of the resistor elements 8 arecompressed, and hence the resistance, which is the property of theresistor, is largely varied. When the stimulation is removed, theresistor portions thus compressed are elongated, and hence theresistance returns to the original value.

The further advantage of the structure in which the resistor elements 8are provided on and the post 6 is bonded to the same surface of theinsulating board 3 is that the stress sensor of the present inventioncan be manufactured by performing mounting operation only on one sidesurface of the insulating board 3. The mounting operation mentionedabove includes, for example, operation of providing the resistors 2 andoperation of bonding the post 6 to the surface of the insulating board 3with an adhesive or the like. In the case in which the mounting isperformed on two surfaces of the insulating board 3, while mounting isperformed on one surface of the insulating board 3, a position at whichthe other surface of the insulating board 3 is placed must be controlledunder strict conditions in terms of cleanness, softness, and the like.On the other hand, when the mounting is performed on the same surface ofthe insulating board 3, the control under the strict conditionsdescribed above is not necessary. Another advantage is that thealignment of the resistor elements 8 and the post 6 can be easilyperformed. The positional relationship between the resistor elements 8and the post 6 is a significantly important factor that determining thestress sensor properties.

For example, in FIG. 8, when the position of the post 6 is largelydeviated with respect to the positions of the resistor elements 8, thestress applied to the post 6 is propagated in a different manner to eachof the individual resistor elements 8. In the case in which the post 6and the resistor elements 8 are mounted on different surfaces of theinsulating board 3, when one surface of the insulating board 3 isvisually observed, the other surface thereof cannot be observed. Hence,it is difficult to understand the positional relationship between thepost 6 and the resistor elements 8, and as a result, the positionaldeviation therebetween is liable to occur. However, when the mounting isperformed on the same surface of the insulating board 3, the relativepositional relationship between the post 6 and the resistor elements 8is very easily grasped, and hence the positional deviation is unlikelyto occur. In addition, visual inspection can be easily performed when anelement which was disposed at a deviated position is removed.

In addition, in the structure shown in FIG. 1 or 8, a protection filmcovering at least the resistor elements 8 is preferably provided. Theprotection film is made of a material softer than that for theinsulating board 3, and as the material mentioned above, for example,there may be mentioned a silicone resin material or a rubber-basedmaterial. The soft material has an effect of suppressing decrease inadhesion between the insulating board 3 and the resistor elements 8caused by repeated warping (elongation and contraction) of the resistorelements 8 following the warping of the insulating board 3.

In addition, in the structures shown in FIGS. 1 and 8, a material forthe post 6 can be selected from a metal, a ceramic, a resin, or a fiberreinforced resin. The advantage obtained when a metal such as iron orhigh carbon steel is used for the post 6 is that a stress applied can beaccurately propagated to the resistor elements 8 by the rigidity of themetal. In addition, a first advantage obtained when a resin or a glassfiber reinforced resin is used for the post 6 is that when theproduction thereof is performed, less energy is consumed. For example, atemperature for molding and curing a resin or a glass fiber reinforcedresin is very low as compared to a sintering temperature for a ceramicand a casting temperature for a metal. A second advantage is superiormoldability to that of ceramic and metal. For example, when a post 6having a complicated shape is formed, cracking may occur in ceramicduring a molding or sintering step and in metal during a casting step insome cases. The reason for this is that, during cooling, the rigiditymaterial cannot easily follow the volume contraction thereof caused by atemperature change from a very high temperature to room temperature. Onthe contrary, when a resin of a glass reinforced resin is used, since amelting temperature of a resin is very low as compared to the sinteringtemperature and the casting temperature described above, and inaddition, the rigidity of a resin is low as compared to that of a metalor ceramic, the problem described above may not occur at all.

This post 6 may be used when the stress sensor of the present inventionis applied to a pointing device of a personal computer or amultifunctional and multidirectional switch for various electronicdevices such as a mobile phone, in particular, a compact mobile phone.In the case in which the stress sensor of the present invention is usedas the multifunctional and multidirectional switch described above, inorder to enable an operator to recognize by feeling a direction in whicha stress is to be applied, it is preferable that a cross-sectional shapeof a side surface of the post 6 be polygonal so that each order can betransmitted to the electronic device by applying a stress perpendicularto each flat surface formed on the side surface of the post 6. When thecomplication of forming the post 6 having the cross-sectional polygonalshape described above is taken into consideration, the post 6 ispreferably formed of a resin or a glass reinforced resin as describedabove.

In addition, as a material when a resin is used, in particular,poly(vinyl terephthalate) (PVT) or poly(butylene terephthalate) (PBT) ispreferably used. Since these PVT and PBT have superior rigidity amongresin materials, an advantage is obtained in that a stress applied canbe relatively accurately propagated. In addition, since the heatstability is also superior, even under the conditions at a temperatureslightly higher than room temperature, an advantage in that the rigiditydescribed above is maintained can also be obtained.

In addition, in the structures shown in FIGS. 1 and 8, a materialforming the insulating board 3 may be selected, for example, from amaterial primarily composed of a resin, a metal covered with anon-conductive material, and a ceramic. As the material primarilycomposed of a resin, for example, a phenol resin itself, or a fiberreinforced resin such as a molded body made of a glass fiber filledepoxy resin may be mentioned. As the metal covered with a non-conductivematerial, for example, an iron or an aluminum plate covered with apolyethylene resin may be mentioned. As the ceramic mentioned above, forexample, alumina may be used. In addition to flexibility enough to bewarped to some extent, the insulating board 3 is also necessary to haveboth rigidity and elasticity so as to be able to recover its own shapewhen a stress repeatedly applied thereto is removed, and all thematerials described above by way of example can satisfy the aboverequirements.

The reason the stress sensor having the second structure comprises theresistor elements 8 having the first structure according to the presentinvention as the constituent elements will be described. The stresssensor of the present invention grasps the direction and magnitude of astress applied to the post 6 from the variation in resistance of theresistor elements 8 resulting from the application of the stress. Hence,when the resistor elements 8 are formed in a very different manner fromeach other, problems of the balance and stability of output propertiesof the stress sensor may arise. For example, when the individualresistor elements 8 are resistor elements which are directly trimmed,and the lengths of trimming grooves thereof are largely different fromeach other, an element having a larger groove length has a highersensitivity. In addition, a resistor element 8 having a high sensitivityeasily causes deviation in resistance from the original value by the usefor a long period of time. According to those described above, it ispreferable that the variation in resistance of the individual resistorelements 8 be decreased as small as possible before the formation of thetrimming grooves so that the length of the trimming groove is uniformlyformed. Hence, as the resistor element 8 of the present invention, whenresistor elements having small variation in resistance from theformation thereof are used as the constituent elements, a significantadvantage can be obtained. By the same reason as described above, astress sensor composed of the second structure in combination with oneof structures 1 a to 1 d is more preferable.

In addition, even in the case of a stress sensor having the structure inwhich the adjustment of resistance is indirectly performed, that is, inwhich the individual resistor elements 8 are not directly trimmed andthe trimmable chip resistors 11 are trimmed as described above, when thevariation in length of the trimming grooves of the trimmable chipresistors 11 is large, due to ambient environments, problems of thebalance and stability of output properties of the stress sensor mayarise. For example, the resistance of a trimmable chip resistor 11having a longer trimming groove is likely to be varied by an ambienttemperature. Hence, even in the case in which the resistance adjustmentis performed by using the trimmable chip resistors 11, as is the case ofthe resistor elements 8 of the present invention, when resistor elementshaving a small variation in resistance from the formation thereof areused as the constituent elements, a significant advantage can beobtained.

In addition, in the latter case, the variation in resistance of theindividual resistor elements 8 directly causes the variation in output(sensitivity). As a particular example, the case in which the fourresistor elements.8 forms one stress sensor shown in FIG. 8 will bedescribed. The resistance of resistor element A is set to R, and theresistance of another resistor element B is assumed to be one-half ofthat of resistor element. A, i.e., R/2. When the insulating board 3 iswarped so that the warpage of resistor element A and that of resistorelement B are equal to each other, if the resistance of resistor elementA becomes twice, the resistance of resistor element B also becomestwice. As a result, the resistance of resistor element A becomes 2×R,and the resistance of resistor element B becomes R. Accordingly, thechange in resistance of resistor element A is R, and the change inresistance of resistor element B is R/2. When the same stress is appliedto resistor elements having different resistances, as described above,the rate of change in resistance is equal to each other; however, thedifference of change in resistance between the two resistor elements istwice. In general, a stress sensor using resistor elements as a straingage outputs the change in resistance as the magnitude of a stress.Accordingly, as is the case of the resistor elements 8 of the presentinvention, when resistor elements having a small variation in resistancefrom the formation thereof are used as the constituent elements, asignificant advantage can be obtained.

A third stress sensor of the present invention for achieving the objectsdescribed above is a stress sensor comprising the resistor elements 8provided on a surface of the insulating board 3 having the conductors 9obtained by partly removing a conductor layer of the surface as remainsor by an additive method; and the post 6 bonded to or integrated withone of surfaces of the insulating board 3, in which the direction andmagnitude of a stress applied to the post 6 is grasped from variation inresistance of the resistor elements 8 resulting from the application ofthe stress. In the stress sensor described above, the resistor elements8 are composed of the electrodes for the resistor element 8 formed byfilm formation so as to be electrically connected with the conductors 9and the resistors 2 formed by thick film formation between theelectrodes for the resistor element 8, and the resistors 2 are primarilyin contact with flat portions of the electrodes for the resistor element8.

According to the third structure described above, concerning theelectrodes forming the resistor elements 8, the two reasons, that is,the height is large (first reason), and the surface approximatelyperpendicular to the surface of the insulating board 3 is present(second reason), can be excluded from the structure of the presentinvention, and as a result, the variation in resistance of the resistorelements 8 can be decreased.

Since a thick film electrode 13 shown in FIG. 10 which is formed byscreen printing or the like has not first and second reasons at thecontact surface with the resistor 2 (FIG. 10(b)), the variation inresistance of the resistor elements 8 using the thick film electrodes 13is small. However, in order to further decrease the variation inresistance, the resistor 2 is formed so as to be primarily in contactwith the flat portion of the electrode for the resistor element 8 (thickfilm electrode 13). The reason for this is to avoid the influence of thefirst reason. For example, in FIG. 10(b), the thick film electrode 13 inthe vicinity of the conductor 9 has a surface perpendicular to theinsulating board 3. When the resistor 2 and the conductor 9 are formedclose to each other so as to be brought into contact with each other,the first reason described above will become effective, and hence it isnot preferable.

FIG. 11 is a view for illustrating the meaning of the flat portion tosome extent. The thick film electrode 13, which is the resistor-elementelectrode, is divided into three cross-sectional regions a, b, and c.Region a is substantially similar to the appearance of the conductor 9,and when the resistor 2 is provided in this region, the resistor element8 having the first and the second reasons is obtained. Region b ismostly a flat region, and the height from the surface of the insulatingboard 3 is approximately 10 μm, which is generally obtained by generalthick film printing (screen printing or the like). Hence, when theresistor 2 is provided in this region, the resistor element 8 having nofirst and second reasons is obtained. In region c, although theappearance of the thick film electrode 13 is not flat, the height fromthe surface of the insulating board 3 is less than 10 μm, and inaddition, a smooth slope is formed. Accordingly, when the resistor 2 isprovided in this region, the resistor element 8 having no first andsecond reasons is obtained. In this invention, the “primarily flatportion of the resistor-element electrode” indicates region b and regionc in FIG. 11.

In addition, due to properties of a paste for the resistor 2 to be used,region a may not have a surface perpendicular to the surface of theinsulating board 3 as sown in FIG. 11 and may include a surfaceprimarily composed of an inclined component with respect to the surfaceof the insulating board 3 in some cases. In this case, a region that issubstantially flat and has no first and second reasons is regarded asregions b and c. As of now, it has been empirically understood that whenthe shortest distance between the conductor 9 and the resistor 2 is setto approximately the height of the conductor 9 or more, the resistorelement 8 having no first and second reasons can be obtained.

As described above, by setting the shortest distance between theresistor 2 and the conductor 9 to a predetermined distance (the heightof the conductor 9) or more, even when the film for the resistor 2 isformed by a screen printing method as described above, the variation inamount of a resistor paste and the deviation in position thereof, causedby collision between the conductors 9 and the squeegee, can bedecreased, and as a result, the shape formed between the conductors 9can be stabilized. The reason for this is that the position at which thesqueegee collides against the conductor 9 is at a distance from theposition at which the resistor paste is actually provided. When thethick film electrode 13 is provided by a screen printing method, thecollision between the squeegee and the conductors 9 generates someinfluence; however, the vicinity of the conductor 9 may be primarilyinfluenced, and the vicinity of the contact position with the resistor 2is unlikely to be influenced. The reason for this that since theformation of the thick film electrode 13 in the vicinity of the contactpoint described above is performed at a distance of not less than theshortest distance between the resistor 2 and the conductor 9, theresistor 2 is unlikely to be influenced when formed. This is the same asthat described in the above case in which when the film for the resistor2 is formed, it is unlikely to be influenced. In addition, even when theconnection states between the conductors 9 and the thick film electrodes13 vary to some extent from each other, due to the lower intrinsicresistance thereof, the variation in resistance of the resistor element8 is not substantially influenced.

Another advantage of forming the resistors 2 and the thick filmelectrodes 13, which constitute the resistor elements 8 used as a straingage of a stress sensor, by film formation is a high adhesive strengththerebetween. The adhesion between the conductor 9 and the resistor 2 islow, and when a stress is repeatedly applied many times to the interfacebetween the conductor 9 and the resistor 2 during stress sensoroperation, the probability of separation thereof at the interface cannotbe denied. In contrast, it has been believed that the interface betweenthe resistor 2 and the thick film electrode 13 may not be separated atall even when general use conditions are continued for a long period oftime. In this case, as the resistor 2 and the thick film electrode 13, ametal glaze-based material and a resin-based material are both included.In particular, when the resistor 2 and the thick film electrode 13 areboth made of a resin-based material, high adhesion at the interface,superior response to an applied stress because of elasticity of theresin, and superior recovery when an applied stress is removed can beobtained as compared to those made of other materials. Hence, as amaterial forming the resistor element 8 used as a strain gage of astress sensor, it can be said that the resin-based material is suitablyused.

In the structure of the resistor element described above, when there isa portion at which conductors 9 on the two surfaces of the insulatingboard 3 are connected to each other through a conductive materialprovided inside a through-hole, the height of the conductors 9 maybecome higher than usual in some cases, and in particular, the presentinvention is preferably used. The reason the height of the conductor 9may be increased is as follows. In a manufacturing step of a so-calleddouble-sided circuit board, in order to connect wires on two surfaces toeach other by forming a conductive layer on an inner wall of athrough-hole formed in the insulating board 3, an electroless platingstep is performed, and in the step mentioned above, the electrolessplating layer is formed on portions which are to be formed into theconductors 9.

In the stress sensor having the third structure, as shown in FIG. 8 byway of example, the resistor elements, 8 are provided on two lines,which are along the surface of the insulating board 3 forming theresistor elements 8 and perpendicularly intersect each other at thecenter of a sensor effective region of the surface of the insulatingboard 3, and are provided at substantially the same distance from thecenter, and the post 6 is bonded to or integrated with the surface ofthe insulating board 3 so that the center thereof substantiallycoincides with the center of the bottom surface of the post 6. Inaddition, by this stress sensor described above, the direction andmagnitude of a stress applied to the post 6 can be grasped from thevariation in resistance of the resistor elements 8 caused by elongation,contraction, or compression thereof resulting from the application ofthe stress to the post 6.

An example of the stress sensor having the third structure will bedescribed with reference to FIG. 8. The conductors 9 in contact with theresistors 2 shown in FIG. 8, that is, the circuit pattern electrodes 1,are replaced with the thick film electrodes 13. The insulating board 3is made, for example, of a glass fiber filled epoxy resin. On the bottomsurface of the insulating board 3, four pairs of the circuit patternelectrodes 1 are provided so as to be electrically connected to theconductors 9, and the resistors 2 are provided between the respectivepairs of the thick film electrodes 13, thereby forming the resistorelements 8. The resistor elements 8 are provided on two lines, which arealong the surface of the insulating board 3 and intersect each other atthe center of the surface thereof, and are provided at substantially thesame distance from the center described above. The post 6 having anapproximately square bottom surface is bonded to the top surface of theinsulating board 3 with an adhesive or the like. In this case, thecenter of the bottom surface of the post 6 is placed so as tosubstantially coincide with the center of the surface of the insulatingboard 3. In addition, the L-shaped holes 10 are provided in theinsulating board 3 so that the corners of the L shapes face the centerof the insulating board 3. The role of the holes 10 are the same asdescribed in the case of the stress sensor having the second structuredescribed above.

Since the advantage obtained in the case in which the trimmable chipresistors 11 are provided on the top surface of the insulating board 3which are to be connected to the respective resistor elements 8 inseries is the same as that of the second structure, description thereofis omitted.

In this case, the “center” of the above “center of the sensor effectiveregion” and “center of the bottom surface of the post 6” does notstrictly mean the center point and includes deviation from the centerpoint in which the stress sensor effectively functions. The meanings ofthe terms used for explaining the stress sensor having the thirdstructure are equivalent to those used for the stress sensors havingstructures 1 a to 1 d and the second structure. In addition, of course,the combination of the third structure with the second structure andstructures 1 a to 1 d is not denied. The combination described above ismore preferable since the advantages thereof may be favorably enhanced.

In the structure of the third stress sensor shown in FIG. 8, the holes10, the support holes 12, and the trimmable chip resistors 11 areparticularly optional constituent elements (not essential elements) ofthe stress sensor of the present invention. Even when those describedabove are included in the constituent elements, the shape of the hole 10is not limited to an L shape, and the positions at which the supportholes 12 are provided are not limited to the four corners of theinsulating board 3 having a square outline. The shape of the hole 10 maybe optionally changed, for example, into a circular, a square, or arounded square shape, in accordance with design limitation, requiredfunctions, applications, and the like. In addition; the support holes 12may be provided at approximately center of each side of the squareinsulating board 3 shown in FIG. 8.

In the stress sensor having the third structure shown in FIG. 8, thebottom surface of the post 6 and part or the entirety of the resistorelements 8 may also overlap each other without the insulating board 3provided therebetween. The advantage obtained in this case is the sameas that of the stress sensor having the second structure. In addition,by the same reason as that of the stress sensor having the secondstructure, in the third structure shown in FIG. 8, a protection filmcovering at least the resistor elements 8 is preferably provided. Theadvantage of the stress sensor having the third structure which isformed of the resistor elements 8 is the same as that of the stresssensor having the second structure from the viewpoint that the resistorelements 8 have a small variation in resistance from the formationthereof.

The second structure of the resistor element 8 according to the presentinvention for achieving the objects described above comprises:electrodes composed of parts of the conductors 9 on a surface of theinsulating board 3 obtained by partly removing a conductive layer on thesurface as remains or by an additive method; and the resistor 2 formedby film formation between a pair of the circuit pattern electrodes 1 onthe surface of the insulating board 3, wherein the resistor 2 cover twoends of the pair of the circuit pattern electrodes 1 in the widthdirection. In this case, the electrode width direction is the directionalong the surface of the insulating board 3 and perpendicular to acurrent flowing direction when the current passes through the resistorelement 8.

Since the resistor element 8 of the present invention has the secondstructure, an exuded part 14 shown in FIG. 12(a) generated in the pastcan be reduced, and hence the variation in resistance of the resistorelement 8 can be decreased. Since the exuded part 14 is formed of theresistor 2, is brought into contact with the circuit pattern electrode1, and is electrically connected to the opposing electrode, theresistance of the resistor element 8 is influenced. The degree of theinfluence is an uncertain factor depending on the amount of the exudedpart 14, the shape thereof, and the like. The reason for this is that itis very difficult to control the amount of the exuded part 14 and theshape thereof as described above. Hence, by eliminating the uncertainfactor as is the structure described above of the present invention,even in the resistor element 8 comprising the electrodes composed of theparts of the conductors 9 on the surface of the insulating board 3obtained as the remains by partly removing the conductor layer on thesurface, and the resistor 2 provided by film formation between the pairof the electrodes on the surface of the insulating board 3, thevariation in resistance of the resistor element 8 can be decreased.

As described above, in the case in which the circuit pattern electrode 1is used, the exuded part 14 is easily generated as compared to the casein which the thick film electrode 13 is formed, and possible reasons forthis will be described. The primary reason is believed that the circuitpattern electrode 1 has a large height and a surface approximatelyperpendicular to the surface of the insulating board 3 as describedabove. That is, for example, in the case in which a thick film resistoris formed by a screen printing method, first, an approximatelypredetermined amount of a resistor paste passed through a mask isprovided between a pair of the circuit pattern electrodes 1. An areaaround the periphery of the circuit pattern electrode 1 becomes aneasy-flow region for the resistor paste. The reason for this is that,around the periphery of the circuit pattern electrode 1, the pasteprovided at the vicinity of the peak of the circuit pattern electrode 1is likely to flow from a higher position to a lower position by its ownweight along the approximately perpendicular surface. Due to this easyflowing property, the flow amount becomes excessive, and as a result,the exuded part 14 is generated.

In the conventional resistor element 8 shown in FIG. 12(b), since thethick film electrode 13 has a small height and a smoothly inclinedsurface with respect to the surface of the insulating board 3, theeasy-flow region for the, resistor paste is not formed on the inclinedsurface, and as a result, the conditions are formed in which the exudedpart 14 is not liable to occur.

Next, by the resistor element 8 having the second structure of thepresent invention, the possibility whether the uncertain factor iseliminated or not will be described. In FIG. 13, an example of theresistor element 8 is shown. The cross-section of this resistor element8 is believed to have approximately the same cross-section as that shownin FIG. 2(a). However, as shown in FIG. 13, when the resistor 2 in theform of paste is provided beforehand at a position at which the exudedpart 14 (FIG. 12(a)) may be generated, even if the paste moves in theeasy-flow region by its own weight from a higher position to a lowerposition along the approximately peripheral surface, an excessive amountof this paste thus moved will be mixed with the resistor 2 pasteprovided at a distance from the surface of the circuit pattern electrode1. Since the amount of the resistor 2 paste generating the exuded part14 is basically small, even when it is mixed with the resistor 2 pasteprovided at a distance, the variation in resistance caused thereby issmall enough to be ignored and cannot be the uncertain factor describedabove. Although the exuded part 14 shown in FIG. 12(a) is small, it hasa function of increasing the area of the interface, in which a currentdensity is high when electricity is applied, between the circuit patternelectrode 1 and the resistor 2, the interface being provided betweenresistor-element electrodes opposing each other. Hence, the resistanceis largely influenced thereby, and as a result, the exuded part 14 isthe uncertain factor as described above. Accordingly, by the structureof the present invention described above, it is clearly understood thatthe uncertain factor can be eliminated.

When the second structure of the resistor element 8 of the presentinvention comprises a portion at which circuit patterns on two surfacesof the insulating board 3 are connected to each other through aconductive material provided inside a through-hole, and the resistor 2formed by film formation between a pair of electrodes on the surface ofthe insulating board 3, the electrodes formed of parts of the conductors9 on the surface thereof, the height of the electrode may become largerthan usual in some cases, and in particular, the present invention ispreferably applied. The reason the height of the electrode may be largeris as follows. In a manufacturing step of a so-called double-sidedcircuit board, in order to connect wires on two surfaces to each otherby forming a conductive layer on an inner wall of the through-holeformed in the insulating board 3, an electroless plating step isperformed, and in the step mentioned above, the electroless platinglayer is formed on portions which are to be formed into the circuitpattern electrodes 1.

A stress sensor having a fourth structure of the present inventioncomprises the resistor elements 8 having the second structure of thepresent invention or the preferable structure based thereon as a straingage, and the post 6 bonded to or integrated with one of surfaces of theinsulating board 3, wherein the direction and magnitude of a stressapplied to the post 6 is grasped from the variation in resistance of theresistor elements 8 resulting from the application of the stress.

In the stress sensor described above, for example, as shown in FIGS. 1and 8, the resistor elements 8 are provided on two lines, which arealong a surface of the insulating board 3 forming the resistor elements8 and perpendicularly intersect each other at the center of a sensoreffective region of a surface of the insulating board 3, and which areprovided at substantially the same distance from the center, and thepost 6 is bonded to or integrated with the surface of the insulatingboard 3 so that the center thereof substantially coincides with thecenter of the bottom surface of the post 6. In addition, by this stresssensor described above, the direction and magnitude of a stress appliedto the post 6 can be grasped from the variation in resistance of theresistor elements 8 caused by elongation, contraction, or compressionthereof resulting from the application of the stress.

The operation and the advantages of the stress sensor having the fourthstructure are the same as those of the third stress sensor. In addition,for example, a modified embodiment in which the trimmable chip resistors11 are used or the like, similar to that of the third embodiment, may beperformed. The meanings of the terms used for the stress sensor havingthe fourth structure are equivalent to those used for the stress sensorshaving structures 1 a to 1 d, the second structure, and structure 3. Inaddition, of course, the combination of the fourth structure withstructures 1 a to 1 d, the second, and the third structures is notdenied. The combination described above is more preferable since theadvantages thereof may be favorably enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of a layout of conductors 9 of astress sensor according to the present invention.

FIG. 2(a) is a schematic side view showing a screen printing step.

FIG. 2(b) is a schematic side view showing the screen printing stepshown in FIG. 2(a) when viewed from between a screen and a board at adifferent angle.

FIG. 3 is a view showing an example of operation of a stress sensoraccording to the present invention.

FIG. 4 is a view showing an overview of an input and output state ofelectrical signals of a stress sensor according to the presentinvention.

FIG. 5 is a view showing an example of a layout of conductors 9 of astress sensor out of the scope of the present invention.

FIG. 6 includes views for illustrating “one end” according to thepresent invention.

FIG. 7(a) is a cross-sectional view of a resistor element composed ofcircuit pattern electrodes;

FIG. 7(b) is a cross-sectional view of a resistor element composed ofthick film electrodes; and

FIG. 8 includes views showing an example of a stress sensor of oneembodiment according to the present invention.

FIG. 9 is a view showing dimensional measurement positions of thedistance (L) between electrodes and the height (h) of the electrode.

FIG. 10(a) is a top view of a resistor element forming a stress sensorhaving a third structure according to the present invention, and

FIG. 10(b) is a side view thereof.

FIG. 11 is a view for illustrating an important portion of the stresssensor having the third structure according to the present invention.

FIG. 12 is a view for illustrating the generation of an exuded part of aresistor in a resistor element.

FIG. 13 is a top view of a resistor element having a second structure ofthe present invention, which forms a stress sensor having a fourthstructure according to the present invention.

FIG. 14 is a top view of a resistor element having the second structureof the present invention, which forms the stress sensor having thefourth structure according to the present invention.

FIG. 15 is a view showing a layout of conductors 9 and the like of aconventional stress sensor.

Reference numerals in the figures indicate as follows, 1 - - - circuitpattern electrode, 2 - - - resistor, 3 - - - insulating board, 5 - - -board terminal parts 6 - - - post, 7 - - - print accuracy adjustingmember, 8 - - - resistor element, 9 - - - conductor, 10 - - - hole,11 - - - trimmable chip resistor, 12 - - - support hole, 13 - - - thickfilm electrode, 14 exuded part, 20 - - - board, 22 - - - resistorelement, 24 - - - board terminal part, and 30 - - - post.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, with reference to figures, an example of embodiments ofstress sensors having structures 1 a to 1 d according to the presentinvention will be described, the stress sensor having a molded body of aglass fiber filled epoxy resin as a board (1.2 mm thick).

First, after copper foils 18 μm thick are adhered to both surfaces of aninsulating board 3, known etching treatment is performed for the copperfoils except for necessary portions thereof, thereby forming conductors9, resistor-element electrodes (circuit pattern electrodes 1), and boardterminal parts 5. A layout of the conductors 9 and resistor-elementelectrodes on a surface of the insulating board 3 of one stress sensorthus formed is shown in FIG. 1. In this case, the side at which theresistor elements are formed is only shown; however wires made ofconductors are also provided on the rear surface of this insulatingboard 3.

On the surface of the insulating board 3, the conductors 9 (printingaccuracy adjusting member 7) not functioning as wires are also formed bythe etching treatment described above. By the presence of these printingaccuracy adjusting members 7, for all four resistor elements 8, thearrangements of the conductors 9, the resistor-element electrodes, andthe printing accuracy adjusting members 7, in the vicinities of therespective resistor elements 8, are similar to each other. In addition,for all the four resistor elements 8, the arrangements of the conductors9, the resistor-element electrodes, and the printing accuracy adjustingmembers 7, in the vicinities of the respective resistor elements 8, areeach formed so as to surround three sides of each of the respectiveresistors 2.

Next, a conductive material is supplied by an electroless plating methodon inner walls of through-holes provided in the insulating board 3beforehand, and hence the conductors on the front and the rear surfacesof the insulating board are electrically connected to each other. InFIG. 1, this position is shown as a “through-hole portion”. In thisstep, the conductive material deposited by this electroless plating isalso deposited on the surfaces of the conductors 9, the resistor-elementelectrodes, and the printing accuracy adjusting members 7, and hence theindividual conductors 9, resistor-element electrodes, and printingaccuracy adjusting members 7 have heights of 30 to 50 μm, which areapproximately equivalent to each other.

Subsequently, a carbon resin-based resistor paste is supplied betweenthe resistor-element electrodes (circuit pattern electrodes 1) by screenprinting. In this step, the moving direction of the squeegee is inclinedat approximately 45° with respect to the insulating board 3 shown inFIG. 1. The resin is then cured by heating, and hence the resistors 2are obtained. In addition, in order to protect the resistors 2, aprotection film formed of a silicone-based resin not shown in the figureis provided by a screen printing method so as to at least cover theresistor element, followed by heat curing.

Next, a columnar post 6 having a square bottom surface is fixed to therear surface of the insulating board 3 with an epoxy resin-basedadhesive. In this step, each side (outline of the bottom surface of thepost 6) of the square bottom surface of the columnar post 6 is disposedat a position corresponding to the respective resistor elements providedon the front surface of the insulating board 3.

Furthermore, trimmable chip resistors (R1trim to R4trim) to beelectrically connected to the respective resistor elements (R1 to R4) inseries are mounted on the rear surface of the insulating board 3 so asto form the connection state shown in FIG. 4. This mounting is performedby using a known electronic component-mounting technique. Subsequently,resistance adjustment is performed for the trimmable chip resistors bylaser trimming so that sets of the resistor elements and the respectivetrimmable chip resistors connected in series have substantially the samesum of resistances. The set described above is formed of the numeralscorresponding to each other in a way such that R1 corresponds to R1trim.

By the steps described-above, the stress sensor of the present inventioncan be obtained. This stress sensor is used while the end portions ofthe insulating board 3 are generally fixed, and in particular, when theinsulating board 3 has a square shape, the four corners thereof arefixed. In FIG. 3, the movement is briefly shown in the case in which astress in the lateral direction is applied to the post 6 when the stresssensor is used. The insulating board 3 in contact with the bottomsurface of the post 6 is not substantially warped, an area in thevicinity of the periphery of the bottom surface of the post 6 ismaximally warped, and an area outside thereof is warped to some extent.

In FIG. 4, an overview of an input and output state of electricalsignals of the stress sensor according to the present invention is alsoshown. Four sets each composed of the resistor element and a trimmablechip resistor 11 form a bridge circuit. Between voltage applicationterminals (Vcc)-(GND) of this bridge circuit, a predetermined voltage isapplied. In addition, the resistor elements, the trimmable chipresistors, and a Y terminal (Yout), provided at the left side in thefigure, form a stress sensor in the Y axis direction, and the resistorelements, the trimmable chip resistors, and a X terminal (Xout),provided at the right side in the figure, form a stress sensor in the Xaxis direction.

In FIG. 5, a surface layout of the insulating board 3 of a stress sensor(hereinafter referred to as “stress sensor B”) out of the scope of thepresent invention is shown. In this sensor, the print accuracy adjustingmembers 7 shown in FIG. 1 are not present. In addition, the arrangementsof the conductors 9 and the resistor-element electrodes (circuit patternelectrodes 1), in the vicinities of the respective four resistorelements 8, are not equal and not similar to each other. In addition,concerning two out of the four resistor elements, each of thearrangements of the conductors 9 and the resistor-element electrodes, inthe vicinities of the corresponding resistor elements, is not formed soas to surround three sides of the periphery of each resistor 2.

(Experiments)

Comparative experiments were performed for the stress sensor of thepresent invention and stress sensor B. The manufacturing conditions forboth sensors were entirely equivalent to each other except for thesurface layout of the insulating board 3. The experimental (evaluation)item was the variation in resistance of individual resistor elementsafter the formation thereof. The variation in resistance of 30 samplesfor each stress sensor, that is, 120 resistor elements for each stresssensor, were shown by the standard deviation. The stress sensor of thepresent invention showed 41.5 Ω, and stress sensor B showed 57.3 Ω. Inaddition, in the stress sensor of the present invention, the variationin shape of the resistors in one stress sensor was not substantiallyobserved, and on the other hand, in stress sensor B, the variation inresistance in one stress sensor was approximately equivalent to thestandard deviation mentioned above. From these results, it is apparentthat the variation in resistor shape in one stress sensor could besuppressed.

Next, with reference to figures (particularly FIG. 8), an example of anembodiment will be described relating to a resistor element having thefirst structure and a stress sensor having the second structure,according to the present invention.

A double-sided copper-clad laminate is prepared in which a copper foilapproximately 18 μm thick used as a conductor layer is provided on eachof two surfaces of a laminate primarily composed of a glass fiber filledepoxy resin having a thickness of 0.8 mm. The front and the rearsurfaces of this double-sided copper-clad laminate is patterned to forma great number of the insulating boards 3 having an approximately squareshape, each regarded as one unit shown in FIG. 8. By this patterning,the insulating boards 3 are formed in the longitudinal and the lateraldirections so that in each of the insulating boards 3, wires 7 and thecircuit pattern electrodes 1 are formed and that the resistor elements 8and the trimmable chip resistors 11 are electrically connected to eachother at the last as shown in FIG. 4. In a first step of thispatterning, holes are formed at positions at which conductive paths arerequired for electrical connection between the front and the rearsurfaces of the double-sided copper-clad laminate. In a second step, inorder to electrically connect the copper-foils on the front and the rearsurfaces to each other by forming conductors on inner walls of thethrough-holes thus formed, electroless copper plating with a catalystand electrolytic copper plating are performed in that order. In thisstep, on the copper foils on the two surfaces of the board, copper isdeposited by plating, and as a result, the total thickness of copper onthe surface of the board becomes is approximately 50 μm. From a thirdstep, the conductor layer on the front surface is partly removed by aknown photoetching method using a dry film resist. As the remains of theconductor layer, the wires 7 and the circuit pattern electrodes 1 areobtained. After the steps described above, the distance (L) of a pair ofthe circuit pattern electrodes 1 is 1.2 mm. Hence, the ratio L/h is 24.

Next, the large insulating board thus formed is roll-pressed so as toadjust the circuit pattern electrodes 1 to have a height of 30 μm. Bythis step, the ratio L/h becomes 40. Next, each insulating board 3,which is one unit, is processed by punching to form the holes 10 shownin FIG. 8.

Next, a resistor paste formed of a thermosetting resin (carbon resinbase) is provided between the circuit pattern electrodes 1 by screenprinting followed by heating for curing, thereby forming the resistors2. Furthermore, in order to protect the resistors 2, a silicone-basedresin paste is screen-printed and is then cured, thereby forming aprotection film. By the steps described above, the resistor elements 8having the first structure of the present invention are obtained.

Next, the trimmable chip resistors 11 electrically connected with theseresistor elements 8 in series by wiring are disposed by a know mountingtechnique and a reflow technique so as to realize the connection statewith the resistors 2 as shown in FIG. 4. In addition, as shown in FIG.8, the trimmable chip resistors 11 are disposed on the opposite surfaceof the board 3 from that for the resistor elements 8 and on thenon-deformable portions described above.

Next, in order to adjust the sum of the resistances of the resistorelement 8 and the trimmable chip resistor 11, electrically connected tothe corresponding resistor element 8, to a predetermined range, lasertrimming is performed for each of the trimmable resistors 11. The reasonthe trimming is not performed for the resistor 2 directly forming theresistor element 8 is that the prevention of destabilization inresistance is considered, which may occur when the resistor 8 made of aresin and the board 3, primarily formed of a resin and provided with theresistors 2, are processed by trimming. These resins show unstablebehavior when processed by treatment, such as trimming, performed at avery high temperature.

Whether the trimmable chip resistors 11 are used or not should bedetermined in consideration of materials of individual members formingthe resistor element 8 and materials for the insulating board 3. Forexample, when the material for the insulating board 3 is a ceramic, andthe material for the resistor 2 is a metal glaze, even if the resistor 2directly forming the resistor element 8 is processed by laser trimming,inconvenience such as resistance destabilization thereafter may be smallenough to be ignored. Hence, in the case described above, the trimmablechip resistors 11 may not be used. However, when it is necessary to usethe trimmable chip resistors 11 by some other reason, of course, theyshould be used in accordance with the necessity.

In addition, as shown in FIG. 8, to each unit, that is, to eachinsulating board 3, a post 6 made of a PBT molded part having a squarebottom surface is fixed with an epoxy adhesive so that the bottomsurface thereof is in contact with the opposite surface of theinsulating board 3 from that on which the resistor elements 8 areprovided and that the center of the bottom surface substantiallycoincides with the center of corresponding one unit of the insulatingboard 3. Accordingly, a set of a plurality of the stress sensors,according to the present invention, is obtained.

Next, in order to obtain the individual units, that is, the individualinsulating boards 3 from the large insulating board, the largeinsulating board is cut and divided by a disc cutter along a pluralityof dividing lines (visible or invisible lines may be used) formed on thesurface of the large insulating board in the longitudinal and thelateral directions, and hence the individual stress sensors areobtained. When the posts 6 are fixed as in this example before thedivision is performed, the workability is improved. The reason for thisis that operation for bonding the posts 6 to the divided and individualinsulating boards 3 each having the stress sensor is complicated andinferior to that performed for the large insulating board 3 in terms ofhandling qualities and properties.

When the large insulating board is made of a ceramic such as alumina, alarge insulating board provided with a great number of dividing groovesin the longitudinal and the lateral directions beforehand is preferablyused. The reason for this is that without using a disc cutter, thedividing operation can be easily performed by applying a force with handor the like so as to open the dividing grooves.

The stress sensor of the present invention is used, for example, whilebeing fixed to a housing or the like of an electronic device using thesupport holes 12 shown in FIG. 8. In this fixed state, the peripheralportions of the insulating board 3, which are outside the holes 10,become non-deformable portions that are not substantially deformed evenwhen a stress is applied to the post 6. In addition, the inside of theholes 10 is deformed when a stress is applied to the post 6 and hencebecomes a deformable portion which elongates and contracts the resistorelements 8. This deformable portion serves as a “sensor effectiveregion” on the surface of the insulating board 3.

FIG. 4 shows an overview of an input and output state of electricalsignals of the stress sensor having the second structure. The state isthe same as that of each of the stress sensors having structures 1 a to1 d.

In the state in which the stress sensor is fixed to the housing, when agap is present under the bottom surface of the stress sensor, a stressapplied downward (in Z direction) to the post 6 can be detected. Thereason for this is that by applying a stress downward as describedabove, all the four resistor elements used as a strain gage areelongated, and as a result, the individual resistances can be increasedto approximately the same value. Since being different from electricalproperties obtained when a stress is applied in the lateral direction (Xdirection or Y direction), the electrical properties described above canbe discriminated therefrom.

In the stress sensor, when some function is created by using this stressapplication to the downside (z direction), multifunctionality can beenhanced. For example, when the stress sensor of the present inventionis used as a pointing device of a computer, a so-called mouse-clickingfunction may be served by the stress application toward the downsidedescribed above. In addition, for example, when the stress sensor of thepresent invention is used as a multifunctional and multidirectionalcompact mobile device such as a mobile phone, stress application to thedownside for a predetermined time may correspond to the instruction onON and OFF operation of a power source of the mobile device.

In this example, in order to partly remove the copper foil used as aconductor layer on the front surface, a photoetching method using a dryfilm resist is used; however, an ED (Electro Deposition) method fordepositing a photoresist by electrophoresis may be used instead. Inaddition, as means for forming the conductors 9 and the circuit patternelectrodes 1 shown in FIG. 8, instead of the method in which theconductor layer on the front surface is partly removed, of course, aso-called additive method can be used in which copper is grown byelectroless plating on the front surface (including inner wall surfacesof through-holes) of the insulating board 3 followed by patterning.

Next, with reference to figures, an example of an embodiment will bedescribed relating to a resistor element having the second structure anda stress sensor having the third structure, according to the presentinvention.

A process for forming the circuit pattern electrodes 1 and theinsulating board 3, composed of a molded body of a glass fiber filledepoxy resin, from the start to the step of forming the resistors byscreen printing is the same as that for the example of the embodiment ofthe resistor element having the second structure. In a subsequent stepin which a thermosetting resin-based (carbon resin based) resistor pasteis provided by screen printing between the circuit pattern electrodes 1followed by heating for curing so as to obtain the resistors 2, theinterval of the circuit pattern electrode 1 is set to 1.2 mm, the widthof the resistor 2 is set to 1.6 mm, and as shown in FIG. 13, theresistor 2 is provided so as to cover two ends of the circuit patternelectrode 1 in the width direction. In addition, the entire top surfaceof the circuit pattern electrode 1 is covered with the resistor 2. Thelength of the resistor 2 protruding from each of the circuit patternelectrodes 1 to the conductor 9 (FIG. 8) side is set to approximately0.2 mm.

Subsequently, in order to protect the resistors 2, a silicone-basedresin paste is screen-printed followed by curing thereof, therebyforming the protection film. By the steps described above, the resistorelement 8 having the second structure of the present invention can beobtained.

By performing subsequent steps of forming the stress sensor, which arethe-same as those for the stress sensor having the second structure, thestress sensor having the third structure can be obtained.

In this example, instead of a photoetching method using a dry filmresist, of course, an ED method or an additive method may also be used.

In FIG. 14, another example of an embodiment of the resistor element 8of the present invention is shown. In a manner different from that shownin FIG. 13, the entire both ends in the width direction of the circuitpattern electrode 1 viewed from the above are not covered, and parts ofboth ends in the width direction of the circuit pattern electrode 1remain uncovered. In this case, an exuded part 14 is formed at aposition different from that shown in FIG. 12(a) by the same mechanismas that in the case shown in FIG. 12(a). Even when the exuded part 14,the amount and the shape of which are not easily controlled, isgenerated at the position described above, the influence thereof on theresistance of the resistor element 8 is small enough to be ignored. Thereason for this is that since this exuded part 14 is a very minuteuncertain factor which is generated in the region other than that of theresistor 2 (the region in which a highest current density is obtained)in which two circuit pattern electrodes 1 oppose each other.Accordingly, since the resistor elements 8 shown in FIG. 14 achieve theobjects of the present invention, they can be regarded as the example ofthe embodiment of the resistor element 8 of the present invention.

Next, with reference to figures, an example of an embodiment of thestress sensor having the fourth structure of the present invention willbe described.

A process for forming a circuit pattern and the board, composed ofmolded body of a glass fiber filled epoxy resin, from the start to thestep of forming the resistors by screen printing is the same as that forthe example of each embodiment of the resistor elements having thesecond structures. However, the circuit pattern electrodes 1 are notformed, and thick film electrodes are formed instead as described below.

A thermosetting resin-based (silver resin based) conductive paste isscreen-printed followed by heating for curing to form thick filmelectrodes 13 while being in contact with the circuit pattern as shownin FIG. 10. Next, a thermosetting resin-based (carbon resin based)resistor paste is applied between pairs of the thick film electrodes 13followed by heating for curing to form the resistors 2. In this step,the thick film electrode 13 and the resistor 2 are formed so as to be incontact with each other in regions b and c as shown in FIG. 11. Inaddition, in order to protect the resistors 2, a silicone-based resinpaste is screen-printed followed by curing thereof, thereby forming theprotection film.

By performing subsequent steps of forming the stress sensor, which arethe same as those for each of the stress sensors having the second andthe third structures, the stress sensor having the fourth structure canbe obtained.

In this example, as the electrode for the resistor element 8, the thickfilm electrode 13 is used; however, instead thereof, the electrode forthe resistor element 8 may be formed by a thin film technique such assputtering, deposition, or plating. When the thickness thus formed is ina common range (several micrometers), the resistor element 8 having nofirst and second reasons described above can be obtained, and as aresult the objects of the present invention can be achieved. Inparticular, in the case in which a step of plating copper on inner wallsof the through-holes of the insulating board 3 is performed as in thisexample, the electrodes for the resistor element 8 can also be formed atthe same time. Hence, since the stress sensor of the present inventioncan be obtained without performing the step of forming the thick filmelectrodes 13 as in this example, the process described above isbelieved to be preferable.

INDUSTRIAL APPLICABILITY

According to the present invention, even in a resistor elementcomprising electrodes composed of parts of conductors obtained asremains by partly removing a conductive layer on a surface of aninsulating board, and resistors formed between pairs of the electrodesprovided on the surface thereof, the variation in resistance of theresistor elements can be decreased. In addition, a stress sensor usingthe resistor elements in which the variation in resistance thereof isdecreased can also be provided.

The stress sensor described above can be preferably used, for example,for a pointing device of a personal computer and a multifunctional andmultidirectional switch for various electronic devices.

In addition, the stress sensor described above can be preferably appliedto a stress sensor which uses a board formed of a molded plate of aglass fiber filled epoxy resin and which can reduce the cost as comparedto that in the past.

1. A method for manufacturing a stress sensor in which the direction andmagnitude of a stress applied to a post bonded to or integrated with asurface of an insulating board can be grasped from variation inresistance of a plurality of resistor elements caused by stimulationresulting from the application of the stress, the method comprising: afirst step of forming resistor-element electrodes, board terminal parts,and conductors so that the resistor-element electrodes are connectedthrough the conductors to the board terminal parts provided at one endof the insulating board; a second step of providing an insulating filmon a surface of the insulating board so as not to cover at least theresistor-element electrodes; and a third step of forming resistors by ascreen printing method between pairs of the resistor-element electrodesdisposed on a surface of the insulating board, wherein the first step,the second step, and the third step are performed in that order.
 2. Amethod for manufacturing a resistor element, comprising: a first step ofobtaining conductors on a surface of an insulating board; a second stepof positively adjusting the height of parts or the entirety of theconductors; and a third step of, by using parts of the conductors aselectrodes, forming a resistor by film formation between a pair of theelectrodes provided on the surface of the insulating board, in which thefirst to the third steps are performed in that order, wherein, in thesecond step, the ratio L/h of the distance (L) between said pair of theelectrodes to the height (h) of the conductors is set to 30 or more, orsaid h is set to 0 or less.
 3. The method for manufacturing the resistorelement, according to claim 2, wherein the first step is a step ofobtaining the conductors by removing a conductive layer on the surfaceof the insulating board or by an additive method.
 4. The method formanufacturing the resistor element, according to claim 3, furthercomprising, between the first step and the second step, a step ofperforming plating treatment inside a through-hole in the insulatingboard.
 5. The method for manufacturing the resistor element, accordingto claim 4, wherein the second step is a step of pressing the surface ofthe board.
 6. The method for manufacturing the resistor element,according to claim 4, wherein the second step is a step of polishing thesurface of the insulating board or a step of performing acidic treatmentthereof.
 7. The method for manufacturing the resistor element, accordingto claim 4, wherein the second step is a plating step of performingplating treatment inside a through-hole in the insulating board aftersaid pair of the electrodes on the surface thereof is covered.
 8. Amethod for manufacturing stress sensors, comprising: step A of formingresistor elements, each corresponding to the resistor element formed bythe manufacturing method according to claim 7, in which sets of theresistor elements, each set functioning as a stress sensor, are providedin respective regions surrounded by a great number of longitudinal andlateral dividing lines formed in a large insulating board; step B offixing posts at positions on the surface of the board corresponding tosaid sets of the resistor elements; and step C of dividing the largeinsulating board along the dividing lines so as to obtain the individualstress sensors, wherein step A, step B, and step C are performed in thatorder.